1. Field of the Invention
The present invention relates generally to methods and apparatus for sensing light energy using light sensitive devices such as charge-coupled device imaging arrays (CCD), and in particular to methods and apparatus for accurately detecting light signals under conditions where low signal-to-noise ratios exist (e.g., relatively low optical power received within relatively short time intervals in the presence of ambient illumination).
2. State of the Art
Devices and systems for detecting transmitted light energy are relatively well known and are used, for example, in laser radar systems. In laser radar systems a reflected pulse of light energy (e.g., laser pulse) is detected by a light sensitive device (e.g., charge-coupled device imaging array). The lateral position of the detected pulse on an array of the charge-coupled device can be used to provide information regarding the position of an object which reflected the pulse. However, conventional light signal detection devices often encounter difficulty in accurately detecting the reflected laser pulse when its total energy is relatively low in comparison with unwanted ambient illumination which is accumulated by the imaging array along with the laser pulse energy.
Ambient illumination caused by any natural or artificial light source in a vicinity of the imaging array can produce background noise. For example, ambient illumination due to sunlight, room light or shot noise from the laser pulse emission, can produce background illumination. As the relative energy of the reflected laser pulse (i.e., determined by optical power and pulse duration) is decreased, conventional detection devices have difficulty distinguishing the laser pulse from background illumination. Because background illumination is continuous and generally non-uniform across imaging array, small fluctuations in the background illumination can be improperly detected as the laser pulse. Further, large fluctuations in background which occur slowly over time with respect to the laser pulse duration (e.g., background fluctuations on the order of milliseconds relative to laser pulse durations on the order of nanoseconds) can result in mischaracterization of background noise as the incident signal.
Conventional techniques for discriminating background illumination from the laser pulse typically acquire an accumulated first signal from the imaging array which represents a combination of the laser pulse superimposed on the background illumination. Either prior to or soon after acquiring the superimposed signal, a second signal can be obtained from the detection device which represents only the background illumination. The second signal representing background illumination can then be subtracted from the first signal in a differential amplifier which is independent of the imaging array, leaving a differential signal assumed to be proportional to the laser pulse energy.
Co-pending U.S. application Ser. No. 08/012,869, and entitled "Dual Mode On-Chip High Frequency Output Structure With Pixel Video Differencing For CCD Image Sensors", Chamberlain et al, the disclosure of which is hereby incorporated by reference in its entirety, describes an output amplifier for use in conjunction with a charge-coupled device to differentially combine a first superimposed laser pulse/background illumination signal and a second background illumination signal. The output amplifier can provide a difference signal without adding complexity to the signal processing scheme of conventional techniques. If the incident light from a laser pulse is confined laterally to a first pixel "n" in a charge-coupled device array, a floating gate amplifier can subtract the background illumination accumulated in a second, separate pixel immediately adjacent the first pixel (i.e., pixel "n -1" or pixel "n +1") thereby leaving only the charge associated with the laser pulse.
Conventional techniques for discriminating a signal of a interest (e.g., laser pulse) from background illumination do not fully address all conditions which affect signal-to-noise ratio. For example, such techniques operate on the assumption that background illumination will not have changed substantially between times at which the first and second signals were acquired (e.g. it is assumed that shot noise due to emission of the laser pulse is negligible). Regardless of whether digital or analog signal processing is used, high-speed electronics required to process the acquired data produce additional system noise which further degrades signal-to-noise ratios.
In addition, conventional techniques do not account for background illumination which is non-uniform across pixels of the charge-coupled device array (e.g., sharp variations in background illumination from pixel to pixel can be falsely detected as a laser pulse). Other factors which can produce non-uniform background illumination from one pixel to another include non-uniform characteristics of the pixels themselves. For example, pixel response non-uniformity (PRNU) or fixed pattern noise (FPN) can cause pixels to produce different electrical signal outputs in response to the same level of illumination). Because signals of interest are not necessarily confined laterally to a single pixel, the effects of pixel response non-uniformity and fixed pattern noise, or the existence of large variations in the background illumination, can result in ambiguous signal detection which becomes aggravated as the signal-to-noise ratio is further degraded.
FIG. 1A illustrates a detection device 100 having an imaging region 118 with four pixels represented as photoelements 110, 112, 114, and 116. The imaging region 118 can be, for example, a full frame, frame transfer, interline transfer (ILT), time-delay and integration (TDI), or frame interline transfer imaging region. Below the imaging region 118 is a horizontal charge-coupled device (HCCD) 120 comprising three-phase stages 122, 124, 126 and 128 for reading-out accumulated photocharge from the photoelements. Those skilled in the art will appreciate that a three-phase stage is a set of three contiguous gates, with a stage being the smallest set of independent horizontal charge-coupled device gates. Because a separate three-phase stage is provided for each photoelement, the lateral pitch of the horizontal charge-coupled device 120 can be considered equal to that of the imaging region 118.
The FIG. 1A detection device further includes an independent output amplifier 130 separate from the imaging region and horizontal charge-coupled device. The output amplifier 130 can obtain a difference between two separate, spatially adjacent photocharge packets stored in adjacent three-phase stages of the horizontal charge-coupled device 120. A laser pulse having an energy "L" is laterally confined to photoelement 112, while a non-uniform background illumination "B" in each of the four photoelements is illustrated by the different background illuminations B1, B2, B3 and B4 in each of the four pixels illustrated. Those skilled in the art will appreciate that the variation in background illumination for each of the pixels of FIG. 1A can, for example, be attributed to non-uniform pixel response, fixed pattern noise, variation in background illumination, or any combination of two or more of these effects. It is further assumed in the FIG. 1A example that the photocharge created by the laser pulse illumination on the photoelement 112 is less than or comparable in magnitude to fluctuations of the background illumination present in each of the pixels.
The output amplifier 130 provides: (1) a first difference between photocharge accumulated in a first pixel 116 and a reset level of the amplifier (i.e., zero amplitude); (2) a second difference between photocharge in the pixels 114 and 116; (3) a third difference between photocharge in the pixels 112 and 114; and (4) a fourth difference between photocharge in the two separate pixels 110 and 112. Thus, conventional imaging arrays cannot accurately detect the existence of photocharge due to the laser pulse in the pixel 112 because the laser pulse energy in combination with the background illumination B3 is comparable to the photocharge accumulated in the remaining pixels. Further, variations in the background illumination at a given pixel may be falsely identified as a laser pulse. Thus, poor signal-to-noise performance can inhibit signal detection.
FIG. 1B illustrates a situation where: B2 is less than B1; (L+B3) is greater than B2; and B4 is greater than (L+B3). The output amplifier 130 will produce four differential values labelled 1-4 in FIG. 1B. However, the imaging array would be unable to distinguish the differential value 3 from the differential value 4. Further, differential value 1 would be significantly greater than either of differential values 3 or 4. Thus, the imaging array 100 would be unable to distinguish the existence of a laser pulse in pixel 112 of FIG. 1A, and may falsely indicate the existence of a laser pulse in pixel 116.
FIGS. 1C and 1D illustrate an aggravated condition which can exist using conventional charge-coupled device imaging arrays when the laser pulse is not confined to a single pixel. As illustrated in FIG. 1C, laser energy L1 from a given laser pulse is incident on pixel 114 (FIG. 1A), laser energy L2 from the same laser pulse is incident on pixel 112 and laser energy L3 from the laser pulse is incident on pixel 110, the laser pulse lateral spatial profile being assumed gaussian. The laser energy is illustrated in FIG. 1C as being superimposed on non-uniform background illumination values B1, B2, B3 and B4 with respect to each of the pixels illustrated. As a result, several smaller intensity photocharge packets of laser energy are distributed in more than one pixel, thereby aggravating the detrimental affects of pixel response non-uniformity, pixel fixed pattern noise and variations in background illumination. Consequently, the output amplifier 130 will produce differential outputs as illustrated in FIG. 1D, wherein the laser pulse intensity differential between adjacent pixels is relatively small such that the laser pulse is not readily detectable.